1. Field of the Invention
This invention relates to line pipe that has good weldability and corrosion-resistance, and more particularly to steel pipe used to transport oil and natural gas, for example, the steel pipe having good resistance to corrosion in an environment that contains wet carbon dioxide and small amounts of wet hydrogen sulfide and the like, and good weldability with respect to circumferential welds performed in the field.
2. Description of the Prior Art
In recent years there has been an increase in the production of oil and natural gas containing wet carbon dioxide and wet hydrogen sulfide. It is a known fact that in such an environment there is marked corrosion of carbon and low alloy steels. To prevent corrosion of line pipe owing to the transportation of oil or natural gas, the normal practice has been to add corrosion inhibitors.
However, in the case of marine pipelines it is very costly to add and recover corrosion inhibitors, and the risk of pollution of the marine environment is also making it increasingly difficult to use corrosion inhibitors. As a result, there is a major need for corrosion-resistant materials that do not require the addition of a corrosion inhibitor.
The use of stainless steel as a corrosion resistant material for applications involving oil and natural gas containing large amounts of carbon dioxide gas is being studied. As described by L. J. Klein in paper number 211 of Corrosion '84, martensitic stainless steels having a carbon content of around 0.2 percent and a chromium content of 12 or 13 percent are widely used as a high strength, relatively low-cost steel, AISI Type 420 steel being a typical example. However, a relatively high carbon content is needed to provide such steels with the necessary strength.
Pipelines are constructed by welding sections of pipe together in the field, but using the usual methods to weld this type of relatively high carbon martensitic stainless steel results in a marked increase in the hardness of the welding heat-affected zones and a degradation in the impact toughness. In addition, when the fluid being transported contains hydrogen sulfide, the increase in the hardness of welding heat-affected zones increases the risk of sulfide stress cracking, thereby degrading the safety of the line pipe.
Following the welding, the hardness of the welding heat-affected zones can be decreased and the toughness improved to some extent by the application of post-weld heat treatment involving heating the steel to at least 600.degree. C. In practice, however, temperature control and quality assurance requirements make it difficult and extremely costly to apply post-weld heat treatment to a line pipe under construction. Thus, there is a need for line pipe that can be welded by normal welding methods without causing much increase in the hardness of zones affected by the heat of the welding, and which also exhibits good low-temperature impact toughness, both at the welding heat-affected zones and of the base metal.
While a reduction in the carbon content of martensitic stainless steels can mitigate the increase in the hardness of the welding heat-affected zones, the resultant coarsening of the ferrite grains in the microstructure of the heat-affected zones produces a pronounced degradation in the impact toughness.